Zeolites are crystallised tectosilicates. Their structures are composed of assemblies of TO.sub.4 tetrahedra which form a three-dimensional network by joining the oxygen atoms. In zeolites of the gallosilicate type, T represents tetravalent silicon and trivalent gallium. The abovementioned three-dimensional network has cavities and channels which are of molecular dimensions and take up cations for compensating the charge deficit owing to the presence of trivalent gallium in the TO.sub.4 tetrahedra, said cations being in general exchangeable.
In general, the compositon of the zeolites can be represented by the empirical formula (M.sub.2/n O, Y.sub.2 O.sub.3, xZO.sub.2) in the dehydrated and calcined state. In this formula, Z and Y respectively denote the tetravalent and trivalent elements of the TO.sub.4 tetrahedra, M represents an electropositive element of valence n, such as an alkali metal or alkaline earth metal, and represents the compensating cation, and x is a number which can range from 2 to theoretically infinity, in which case the zeolite is a silica.
Each type of zeolite has a distinct microporous structure. The change in the dimensions and forms of the micropores from one type to another involves changes in the adsorption properties. Only the molecules of certain dimensions and forms are capable of entering the pores of a particular zeolite. Due to these remarkable characteristic features, zeolites are very particularly suitable for the purification or separation of gaseous or liquid mixtures, such as, for example, the separation of hydrocarbons by selective adsorption.
The chemical composition, including in particular the nature of the elements present in the TO.sub.4 tetrahedra and the nature of the exchangeable compensating cations, is likewise an important factor playing a role in the selectivity of adsorption and in particular in the catalytic properties of these products. They are used as catalysts or catalyst supports in cracking, reforming and modification of hydrocarbons as well as in the production of a large number of molecules.
Many zeolites occur in nature; they are aluminosilicates whose accessibilities and properties do not always meet the requirements of industrial applications. Accordingly, the search for products having novel properties has led to the synthesis of a wide range of zeolites, of which zeolite A (US-A 2,882,243), zeolite X (US-A 2,882,244), and zeolite Y (US-A 3,130,007) may be mentioned in particular.
The zeolites based on a gallosilicate skeleton are also quite numerous, and especially the gallium zeolite of the ZSM-5 type (EP-A 0,266,825), the gallium zeolite of the ZSM-11 type (EP-A 0,223,388) and certainly the gallium zeolites of the faujasite type (US-A 4,803,060 and US-A 3,431,219) may be mentioned in particular.
The zeolites from the structural class of faujasite are distinguished by a three-dimensional network structure, which can be described as an assembly of modules called cubic octahedra. Each of these modules is composed of 24 tetrahedra containing the elements Si and Ga in our case and bridged by oxygen according to the principle described above. In the cubic octahedron, the tetrahedra are linked such that eight rings of six tetrahedra and six rings of four tetrahedra are formed.
Each cubic octahedron is linked via four rings of six tetrahedra to four adjacent cubic octahedra in tetrahedral coordination.
In order to show the relationships which unite the different members of the structural family, it is appropriate to look at the structural planes in which the cubic octahedra are arranged at the vertices of a two-dimensional lattice of hexagons. In the structural plane, each cubic octahedron is also linked to three neighbors.
The fourth bonding direction extends alternately on both sides of the structural plane and allows the linkage of the cubic octahedra between adjacent and parallel structural planes.
All solids belonging to the structural class of faujasite have interconnected channels of about 0.8 nm in diameter. Thus, faujasite is a zeolite with an aluminosilicate skeleton, whose structure corresponds to the stacking of three distinct structural planes, ABC, which corresponds to a structure of cubic symmetry.
Compounds of the same structure as that of faujasite can be obtained by synthesis from a sodium gallosilicate gel. Reference may be made to US-A 3,431,219 and US-A 4,803,060 mentioned, previously which describe the synthesis of zeolites of the faujasite type, which have a gallosilicate skeleton whose Si/Ga ratio is between 1 and 3.
Higher Si/Ga ratios cannot be obtained by synthesis.
The general procedure for the synthesis of zeolites which have a gallosilicate skeleton and belong to the structural class of faujasite consists in hydrothermal crystallisation of sodium gallosilicate gels of specific composition comprising a structuring agent in the form of a metal cation.
More specifically, such a procedure consists first in preparing a reaction mixture having a pH of greater than 10 and comprising water, a source of tetravalent silicon, a source of trivalent gallium, a source of hydroxide ions in the form of a strong base, a source of metal cations M.sup.n+, in which n is the valence of M, in order to obtain a gallosilicate gel having the desired composition for permitting its crystallisation in a compound of the structure class of faujasite, then in maintaining the gel obtained, directly or after preliminary ageing, at a temperature of at most equal to 150.degree. C. and under a pressure of at least equal to the autogenous pressure of the mixture composed of said gel for a sufficient period to effect the crystallisation of this gel.
As mentioned above, such a process does not allow the synthesis of zeolites having a gallosilicate skeleton and the structure of cubic symmetry of faujasite and an Si/Ga ratio of greater than 3.
It has now been found that certain organic molecules belonging to the group of carbon-containing rings and macrocycles containing hetero atoms which are selected from oxygen, nitrogen and silicon, have the property of directing the crystallisation of gallosilicate gels towards zeolites of the structural class of faujasite, which are characterised by Si/Ga ratios which can be greater than 3. Irrespective of the size and symmetry of the ring or macrocycle, a zeolite is obtained having a cubic structure.
On the other hand, the ring or macrocycle provides an important stabilising effect, which makes it possible to reduce the hydroxide ion concentration in the synthetic medium, to the effect that a higher Si/Ga ratio and a substantial improvement in the yield can be obtained.